(1) September 23, 1783: Caroline Herschel discovered the Sculptor Galaxy (NGC 253), a starburst spiral galaxy in the southern constellation Sculptor.
(2) September 23, 1846: Johann Galle discovered Neptune, using Urbain LeVerrier's calculations.
(3) September 27, 2007: NASA's Dawn spacecraft was launched. It has orbited the asteroid Vesta and is still in orbit around the dwarf planet Ceres.
(4) September 30, 2016: ESA's Rosetta mission ended when Rosetta was sent to join Philae on Comet C-G. More about the Rosetta mission: http://www.bellaonline.com/articles/art182574.asp